Re: why there is not VACUUM FULL CONCURRENTLY? - Mailing list pgsql-hackers

Antonin Houska <ah@cybertec.at> wrote:

> Alvaro Herrera <alvherre@alvh.no-ip.org> wrote:
>
> >
> >
> > > From bf2ec8c5d753de340140839f1b061044ec4c1149 Mon Sep 17 00:00:00 2001
> > > From: Antonin Houska <ah@cybertec.at>
> > > Date: Mon, 13 Jan 2025 14:29:54 +0100
> > > Subject: [PATCH 4/8] Add CONCURRENTLY option to both VACUUM FULL and CLUSTER
> > >  commands.
> >
> > > @@ -950,8 +1412,46 @@ copy_table_data(Relation NewHeap, Relation OldHeap, Relation OldIndex, bool verb
> >
> > > +        if (concurrent)
> > > +        {
> > > +            PgBackendProgress    progress;
> > > +
> > > +            /*
> > > +             * Command progress reporting gets terminated at subtransaction
> > > +             * end. Save the status so it can be eventually restored.
> > > +             */
> > > +            memcpy(&progress, &MyBEEntry->st_progress,
> > > +                   sizeof(PgBackendProgress));
> > > +
> > > +            /* Release the locks by aborting the subtransaction. */
> > > +            RollbackAndReleaseCurrentSubTransaction();
> > > +
> > > +            /* Restore the progress reporting status. */
> > > +            pgstat_progress_restore_state(&progress);
> > > +
> > > +            CurrentResourceOwner = oldowner;
> > > +        }
> >
> > I was looking at 0002 to see if it'd make sense to commit it ahead of a
> > fuller review of the rest, and I find that the reason for that patch is
> > this hunk you have here in copy_table_data -- you want to avoid a
> > subtransaction abort (which you use to release planner lock) clobbering
> > the status.  I think this a bad idea.  It might be better to handle this
> > in a different way, for instance
> >
> > 1) maybe have a flag that says "do not reset progress status during
> > subtransaction abort"; REPACK would set that flag, so it'd be able to
> > continue its business without having to memcpy the current status (which
> > seems like quite a hack) or restoring it afterwards.
> >
> > 2) maybe subtransaction abort is not the best way to release the
> > planning locks anyway.  I think it might be better to have a
> > ResourceOwner that owns those locks, and we do ResourceOwnerRelease()
> > which would release them.  I think this would be a novel usage of
> > ResourceOwner so it needs more research.  But if this works, then we
> > don't need the subtransaction at all, and therefore we don't need
> > backend progress restore at all either.
>
> If this needs change, I prefer 2) because it's less invasive: 1) still affects
> the progress monitoring code. I'll look at it.

Below is what I suggest now. It resembles the use of PortalData.resowner in
the sense that it's a resource owner separate from the resource owner of the
transaction.

Although it's better to use a resource owner than a subtransaction here, we
still need to restore the progress state in
cluster_decode_concurrent_changes() (see v07-0004-) because a subtransaction
aborts that clear it can take place during the decoding.

My preference would still be to save and restore the progress state in this
case (although a new function like pgstat_progress_save_state() would be
better than memcpy()). What do you think?


@@ -950,8 +1412,48 @@ copy_table_data(Relation NewHeap, Relation OldHeap, Relation OldIndex, bool verb
      * provided, else plain seqscan.
      */
     if (OldIndex != NULL && OldIndex->rd_rel->relam == BTREE_AM_OID)
+    {
+        ResourceOwner    oldowner = NULL;
+        ResourceOwner    resowner = NULL;
+
+        /*
+         * In the CONCURRENT case, use a dedicated resource owner so we don't
+         * leave any additional locks behind us that we cannot release easily.
+         */
+        if (concurrent)
+        {
+            Assert(CheckRelationLockedByMe(OldHeap, ShareUpdateExclusiveLock,
+                                           false));
+            Assert(CheckRelationLockedByMe(OldIndex, ShareUpdateExclusiveLock,
+                                           false));
+
+            resowner = ResourceOwnerCreate(CurrentResourceOwner,
+                                           "plan_cluster_use_sort");
+            oldowner = CurrentResourceOwner;
+            CurrentResourceOwner = resowner;
+        }
+
         use_sort = plan_cluster_use_sort(RelationGetRelid(OldHeap),
                                          RelationGetRelid(OldIndex));
+
+        if (concurrent)
+        {
+            CurrentResourceOwner = oldowner;
+
+            /*
+             * We are primarily concerned about locks, but if the planner
+             * happened to allocate any other resources, we should release
+             * them too because we're going to delete the whole resowner.
+             */
+            ResourceOwnerRelease(resowner, RESOURCE_RELEASE_BEFORE_LOCKS,
+                                 false, false);
+            ResourceOwnerRelease(resowner, RESOURCE_RELEASE_LOCKS,
+                                 false, false);
+            ResourceOwnerRelease(resowner, RESOURCE_RELEASE_AFTER_LOCKS,
+                                 false, false);
+            ResourceOwnerDelete(resowner);
+        }
+    }
     else
         use_sort = false;



--
Antonin Houska
Web: https://www.cybertec-postgresql.com